The following image comes to us from Avatar_Mom, who is at SDCC right now and went to the poster signing event.

In this amazing image we can see Avatar Korra front and center, sporting a wicked smirk. Behind her and to the left in the poster is a firebender (her firebending teacher? a descendant of Zuko's?), and behind her and to the right in the image is an earthbender (her earthbending teacher?). But where is Tenzin?

In the background we can see figures dressed in all black with glowing green eyes (the anti-benders?), as well as the walls of some tall buildings (this is in Republic City, we can assume). Topping it off, we can see a hooded and masked face looming over everything else in the poster. Is this our main villain? We can't wait to find out!